When I shot a mark V 257 weatherby it would group like that until it was loaded with a hot load then it started to tighten up. I would clean the heck out of the bore and start working up a load from low to high range of powder. If you have other powders ( I can not find my Nosler book at this moment) I would try to see if it liked any thing else with the same type of from low to high charges. oldfamily.